a. Field of the Invention
This invention relates to a heat-developable photosensitive material for an electrostatic printing master, and more particularly, to an improved heat-developable photosensitive material for an electrostatic printing master containing an organic silver salt.
b. Description of the Prior Art
Many printing methods are known. Among them, electrostatic printing methods belong to a special class. The principle of ordinary printing is based on selectively attaching an ink to the surface of a printing master in accordance as a result of an uneven surface on the printing master or as a result of difference of solvent affinity, and then pressing the attached ink to a paper. On the other hand, in the electrostatic printing the ink is replaced by a heat-fixable toner which is electrostatically attached to a printing master and then transferred and fixed to an image-receiving sheet e.g. of paper. Ordinary printing has the advantage that the ink is attached uniformly and firmly to the printing master so that many sheets of paper can be printed at a high speed, but the ink can disadvantageously become attached to portions other than those to be printed. On the other hand, in electrostatic printing methods the toner can become attached electrostatically so that firmness and uniformity of adhesion are heartily dependent upon electrostatic "contrast" which is difficult to achieve so that known methods are not suitable for a high speed printing although dirtying as mentioned above is not such a problem as in ordinary printing. In view of the disadvantages, electrostatic printing has not been practically used as a clean printing method. In other words, electrostatic printing is poorer than conventional printing methods as to providing many sheets of print and uniformly clear print. For example, representative known electrostatic printing masters are a master composed of a conductive support and an insulating image overlying the conductive support and a master composed of an insulating support and a conductive image overlying on the insulating support. The image may be produced by attaching an insulating or a conductive lacquer in a form of the image pattern to the support, or by coating a photosensitive lacquer on a support, imagewise exposing and selectively removing the exposed or unexposed portions by etching. Such electrostatic printing masters have various drawbacks. The sharpness of the print and the durability of the electrostatic printing master are usually poor, e.g. when used in the master is used in a conventional electrostatic printing process. Such a process can include a charging step for forming the electrostatic images by selectively retaining electric charge at image portions (the image portions are insulating), a developing step in which a toner charged with a polarity opposite to that of image portions is applied and a transfer step in which the toner image is transferred to a receiving sheet. For example, the known electrostatic printing master has images formed by unevenness on the surface and the uneven surface is damaged by mechanical abrasion during the printing process to cause irregular charging so that durability of the master is very low. Furthermore it is very difficult to obtain a high resolving power with such an uneven master and thereby it is also difficult technically to obtain a print having high resolution. Furthermore, it is difficult to obtain images of half tone or gradation with such uneven surface type master.
In U.S. Ser. No. 599,061 filed July 25, 1975 there is disclosed a new electrostatic printing master free from the drawbacks of known electrostatic printing masters. The electrostatic printing master has a layer containing silver images carried in an electrically insulating medium having an electric resistance sufficient to retain electrostatic charge and the surface is smooth because there is not any relief patterns corresponding to the images. Therefore, upon printing the images are hardly damaged by mechanical abrasion and have excellent durability, and further the resolution of the silver image itself is high; the resolution is excellent due to a continuous gradation; optical density can be changed in accordance with an optional continuous gradation; and further there are such advantages as very helpful in the fields of printing and high speed copying of many copies. In general, the above-mentioned electrostatic printing master having a layer containing silver images can be obtained by imagewise exposing a silver salt photosensitive material and developing. And the silver images can be formed by a dry treatment so that the photosensitive material containing an organic silver salt is very effective from a point of view of simplicity of image formation.
Such photosensitive materials are usually called "heat-developable photosensitive material". Known heat-developable photosensitive materials are, for example, those as disclosed in U.S. Pat. Nos. 3,457,075; 3,531,286; and 3,589,903.
However, those heat-developable photosensitive materials are for direct copying purpose, that is, the visible images formed on said materials are used for the final purpose. Therefore, those materials can give excellent visible images, but do not have all characteristics necessary for an electrostatic printing master. Thus, those known materials are not satisfactory to producing electrostatic printing masters from a practical point of view.
Printing processes using electrostatic printing master are carried out by, for example, charging the master with a corona discharger to convert the electric resistance patterns to electrostatic charge patterns, developing the electrostatic charge patterns with toner particles to produce toner images and transferring the toner images thus formed to an image-receiving member such as paper.
Therefore, the electrostatic printing master should have the following characteristics, that is, high acceptance potential at portions where electrostatic charge is charged (portions of relatively high electric resistance), good electrostatic charge retentivity, low background potential, high electrostatic potential contrast, high mechanical, electrostatic, repeating durability, excellent property of development, good cleaning property, high sharpness of the final image formed on an image-receiving member, excellent electrical fatigue resistance, sufficient mechanical strength and the like. In addition, it is desirable for the electrostatic printing master to have the following characteristics: simple, rapid and easy production of the electrostatic printing master, simple production of the heat-developable photosensitive material itself, low cost, easy transportation and convenient distribution and the like.